Several methods for making heteroatom-doped carbon have been found in literature. For example, boron (B)-doped carbons have been synthesized using a number of different methods and have recently been evaluated as electrode materials for electric double-layer capacitors (EDLCs) and O2 reduction applications in fuel cells. Similarly, nitrogen (N)-doped carbons have also been evaluated as EDLC electrode materials, along with other applications like CO2 capture and storage.
In one example, a mesoporous B-doped carbon was synthesized by co-impregnation of sucrose and boric acid into a mesoporous silica template (SBA-15), followed by carbonization and etching of the template (Wang, D. W., et. al., 2008). Maximum B-doping levels of 0.6 atomic % ware reported using this method, along with a specific surface area of 660 m2/gm. Specific capacitance of EDLC devices using this B-doped carbon was found to be ˜1.5 times higher than with boron-free carbon, using aqueous electrolytes.
U.S. Pat. No. 7,919,014 described a method to make B-doped activated carbon by mixing an activated carbon powder with a boric acid solution (maximum B % calculated for the mixture was 1.0 atomic %, although the actual atomic % of B in the final carbon was not reported). This mixture was then dried and heated to form the B-doped carbon. It is unclear whether B entered the carbon matrix with this technique or remained within surface functional groups.
Other methods used for synthesizing B-doped carbons include laser-induction and chemical vapor deposition. In the laser induction method (Peng, Z., et. al., 2015), boric acid was dissolved in poly(amic acid) solution, followed by condensation of the solution to form boric-acid-containing polyimide sheets. These sheets were then exposed to a CO2 laser which converted the material into B-doped laser-induced graphene. Capacitance measurements on electrodes using this B-doped graphene resulted in values that were 3 times larger than similar measurements made on electrodes with non B-doped laser-induced graphenes. In another example, chemical vapor deposition (CVD) using benzene, triphenylborane (TPB) as the B source, and ferrocene as a catalyst, resulted in B-doped carbon nanotubes (BCNTs). Boron content was varied by using different TPB concentrations. These materials were used in fuel-cells for oxygen reduction reactions and showed improved performance with increasing boron content (Yang, L., et. al., 2011).
In another example, B-doped graphene nano-sheets (with a maximum of 2.56 atomic % B) were synthesized for use as electrode materials; and capacitance values were compared against similarly synthesized materials that were not doped with B. The capacitance values of the B-doped materials were 2 times higher than the non-B-doped materials, in aqueous electrolytes. In this case, the B-doped graphene nano-sheets were synthesized by first thermally reducing graphene oxide, followed by miring this with boric acid in ethanol and autoclaving at 150° C. (Thirumal, V., et. al., 2016).
There are several examples in the prior art describing methods to make nitrogen (N)-doped carbon from natural sources—due to the existence of precursor materials already rich in nitrogen content. For example, N-doped activated carbons were prepared from chitosan (a biomass precursor obtained from shrimp shells, naturally containing N) (Śliwak, A., et. al., 2016). The process involved high temperature carbonizing, followed by CO2 activation. These carbons were found to have a maximum of 5.4 wt. % N—with specific surface area and specific capacitance values of 1080 m2/gm and 147 F/gm (in aqueous electrolytes), respectively. Similar measurements made using commercially available activated carbon (Berkosorb®), with an N content of ˜0.1 wt. %, exhibited lower capacitance values in aqueous electrolytes, despite having similar surface areas (1101 m2/gm). In a further example, porous nitrogen-doped carbon nano-sheets were prepared via simultaneous activation and graphitization of biomass-derived natural silk. These carbons showed an N-content of 4.7 wt. %, a surface area of 2494 m2/gm and specific capacitance values as high as 242 F/gm, in ionic liquid electrolytes [Hou, J., et. al., 2015].
While the presence of N in the carbons developed from natural sources has been shown to have a beneficial effect on performance (e.g. higher capacitance than non-nitrogen containing carbons with similar surface areas), there is little control over other impurities like Fe, Mn, Ni, Zn, S, Cl, etc., some of which need to be at levels less than 20 ppm for the carbon to be used in commercial EDLC applications.
N-doped carbons have also been made from synthetic starting materials. In one example, a nitrogen-doped porous carbon nanofiber (CNF) structure was synthesized with 4 to 12.14 atomic % N, by mixing the CNF with pyrrole and ammonium persulfate, and carbonizing at temperatures of 1100° C. (Chen, L. F., et. al., 2012). These carbons had specific surface areas of 562 m2/gm and capacitance values that varied with the N-content (7.22 atomic % N showed the best capacitance). In another example, N-doped carbon was synthesized using a soluble resol as a carbon source, dicyandiamide as a nitrogen source, and a surfactant (Pluronic®F127) as a soft template. Following carbonization and pyrolysis (to remove the template), the material was chemically activated using KOH, to obtain a surface area of 494-586 m2/gm. A maximum N-content of 13.1 wt. % was achieved and performance of these carbons for CO2-capture applications (3.2 mmol/gm, at 298K, 1.0 bar) and EDLC applications (262 F/gm in aqueous electrolytes) was measured (Wei J, et. al., 2013).
In yet another example, N-doped carbon was synthesized from the well established urea-formaldehyde condensation reaction by adding furfuryl alcohol to the system prior to co-polymerizing the mixture (Liu, Z., et. al. 2015). This method of co-polymerization involved the polymerization of a furfuryl alcohol and a urea/formaldehyde system—simultaneously. There are several issues with this approach: (i) the kinetics of the urea/formaldehyde condensation reaction are different from the kinetics of the furfuryl alcohol polymerization reaction; (ii) during the carbonizing stage, the volatile organic compounds that are typically released here, are also different. This results in a non-homogeneous distribution of the nitrogen in the final carbon. This can be easily seen from the N wt. % data in this study. Specifically, a doubling of the ratio of furfuryl alcohol to the urea/formaldehyde solution is expected to result in a comparable reduction of the N content—all other process parameters being held constant. However, the published data shows the exact opposite trend, i.e. a reduction in the amount of urea (N-source) shows an increase in the N content of the final carbon. This can only be explained by the co-polymerization process having very different and individual characteristics. Specifically, if the N is not uniformly distributed (i.e. favoring one system over the other), and both these polymerized (albeit intermixed) solids carbonize at different rates, the end result can be an increase in the overall N content with a decrease in the N-source. Consequently, a single polymerization system is sought to ensure uniform distribution of the heteroatoms. Additionally, formaldehyde has been under increasing scrutiny from government and environmental groups due to its carcinogenic nature, and has been listed as a hazardous air pollutant under the US Clean Air Act (amended in 1990). Thus, systems without hazardous air pollutants like formaldehyde are desired.
Recently, N-doped carbon was made using a surfactant-controlled zeolitic imidazolate framework (ZIF-8) [Liu, N. L., et. al., 2016]. A solution of zinc nitrate was added to a solution of 2-methylimidazole and polyvinylpyrrolidone (PVP) at room temperature and aged for 10 hours. The resulting precipitate was washed and dried to form the ZIF-8 and was then carbonized in an inert atmosphere to result in N-doped carbon nano-particles. Measurements of capacitance using aqueous electrolytes resulted in 200 F/gm. A maximum N-doping of 15 wt. % was measured, along with a specific surface area of 798 m2/gm. The various methods of doping carbon with nitrogen and boron described above are summarized in Table 1.
TABLE 1Summary of B- and N-doped activated carbons (prior art).CarbonHeteroatomTargetBET surfaceHeteroatomCapacitance/SourcesourceProcessApplicationsareacontent(electrolyte)Reference1SucroseB = Boric acidTemplatingEDLC<660 m2/gmB = 0.6<125 F/gmWang, D. W., et. al.,atomic %(aqueous)20082ActivatedB = Boric acidThermal dopingEDLCNotB = NotEDLC notU.S. Pat. No.carbonin Solid statemeasuredmeasuredmeasured7,919,0143GrapheneB = Boric acidB-dopedCapacitors 191 m2/gmB = Not6.6 F/mlPeng, Z., et. al.,Laser inducedformeasured(solid state2015grapheneelectronicsPolymer)4BenzeneB = Tri-phenylCVD to makeCatalysts forNotB = <2.24NotYang, L., et. al.,boraneB-doped CNTO2 reductionmeasuredatomic %measured20115GraphiteB = Boric acidThermal dopingEDLCNotB = 2.56113 F/gmThirumal, V., et. al.,in solid statemeasuredatomic %(aqueous)20166ChitosanNaturallyCarbonizationEDLC1080 m2/gmN = 8.3147 F/gmSliwak, A., et. al.,containing Nwt. %(aqueous)20167Biomass-NaturallySimultaneousEDLC2494 m2/gmN = 4.7242 F/gmHou, J., et. al.,derivedcontaining Nactivation andwt. %(Ionic2015natural silkgraphitizationLiquid)8CarbonN = PyrroleThermal dopingEDLC<562 m2/gmN = 12.14202 F/gmChen, L. F., et. al.,Nano fiberand ammoniumin solid stateatomic %(aqueous)2012persulfate9ResolN =TemplatingEDLC<586 m2/gmN = 13.1262 F/gmWei J, et. al.,dicyandiamidewith pyrolysiswt. %(aqueous)2013(DCDA)10Form-N = UreaCo-PolymerizationCO2 capture2273 m2/gmN = 2.93NotLiu, Z., et. al.aldehyde +to 10.33measured2015Furfurylwt. %alcohol11Methyl-N = Methyl-ZeoliteEDLC<800 m2/gmN = 15<200 F/gmLiu, N. L.,et al,imidazole;imidazole;derived nano-wt. %(aqueous)2016Polyvinyl-Polyvinyl-particles(surface)pyrrolidonepyrrolidone
There is need for a new simpler method to make heteroatom-doped carbons from synthetic starting materials. This method should not involve complex processing like templating, laser-processing, CVD, etc. (described in the prior art). Additionally, there is also a desire to eliminate carcinogenic chemicals like formaldehyde (described in the prior art), from the new simpler method.